The present invention relates to a mobile communication system of a cellular scheme such as a automobile telephone system or a portable telephone system, and a base station (BS) and base station controller (BSC) which are used for the mobile communication system.
It is generally known that a radio wave emitted from a given radio station is attenuated in proportion to the power of the distance from the radio station. For this reason, according to the cellular scheme, many base stations are distributed to form cover areas called cells, and each base station takes charge of the mobile stations in each cover area.
In such a cellular scheme, when a mobile station moves between cells during communication, the base station taking charge of the mobile station changes. In this mobile communication system, handoff processing is performed to change the base station that connects the radio path to the mobile station.
In general, handoff processing is performed to connect a radio path between the mobile station and a base station with a higher reception power on the basis of the reception power of a transmission signal received by the mobile station from each base station. More specifically, in a mobile communication system using a CDMA (Code Division Multiple Access) scheme, each base station transmits a pilot channel. Each mobile station measures the reception electric field strength of each pilot channel that can be received, and notifies the base station that connects the radio path to the mobile station of the measurement results. The base station compares the respective pilot reception powers (the reception electric field strengths of the respective pilot signals) received from the mobile station of which the base station is taking charge. If there is another base station that has a pilot reception power higher than the reception electric field strength of the pilot channel transmitted by the above base station, handoff processing is executed to shift the charge of the mobile station to this other base station.
FIG. 1 shows the distribution of pilot reception power associated with adjacent base stations A and B which transmit pilot channels with the same transmission power.
As is obvious from this graph, the pilot reception power of the base station A becomes equal to that of the base station B at a point C as a middle point between the base stations A and B. The pilot reception power of the base station A becomes larger than that of the base station B at any point closer to the base station A than the point C. The pilot reception power of the base station B become larger than that of the base station A at any point closer to the base station B than the point C. Consequently, the handoff execution boundary between the base stations A and B is the point C, i.e., the middle point between the base stations A and B.
When, therefore, base stations are arranged as in FIG. 2, if each base station has its communicative area within the circle indicated by the dashed line in FIG. 2, the cell formed by each base station falls within the hexagonal range indicated by the solid line in FIG. 2.
Since the transmission power of each base station is almost fixed, each cell is fixed to the state shown in FIG. 2. For this reason, even if the traffic in a specific cell is larger than that in an adjacent cell, when a mobile station that has been located in the adjacent cell moves closer to the base station that manages the specific cell, handoff to the base station in the specific cell must be performed. As a result, traffic may concentrate on the specific cell.
For example, a method of locally forming a small cell (micro-cell) at a portion, in a cell, in which the traffic is heavy may be used as a method of preventing traffic concentration. In this case, however, many facilities must be added to the base station, and the coexistence of cells (macro-cells) and micro-cells results in complex, cumbersome control.
As described above, according to the prior art, since the size of each cell is fixed, traffic concentration on a specific cell cannot be avoided.
If micro-cells are formed to prevent such traffic concentration, the base station facilities must be increased, and complex, cumbersome control is required.